Metal-air batteries are comprised of multiple electrochemical cells. Each cell is further comprised of an air permeable cathode and a metallic anode separated by an aqueous electrolyte. Metal-air batteries have a relatively high energy density because they utilize oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material, such as a metal oxide or other depolarizable metallic composition. For example, during discharge of a zinc-air battery cell, oxygen from ambient air is converted at the cathode to hydroxide ions, zinc is oxidized at the anode, reacts with the hydroxide ions, and water and electrons are released to provide electrical energy.
Metal-air cells that are rechargeable and thus useful for multiple discharge cycles are called secondary cells. Electrically rechargeable metal-air cells are recharged by applying a voltage between the anodes and cathodes of the cells and reversing the electrochemical reaction. During recharging, the cell anodes are electrolytically reformed by reducing to the base metal the metal oxides formed during discharge. The electrolytic reformation generates a large amount of oxygen and a small amount of hydrogen which are discharged through the air permeable cathodes and through the vents of the cells, respectively.
Because metal-air batteries use oxygen from ambient air as a reactant in the electrochemical reaction, they provide a relatively light weight and compact power supply. Further, because they are rechargeable, metal-air batteries are an ideal source of power for portable equipment, such as portable computers and telephones.
The anodes are made from metals that can be oxidized during discharge in a metal-air cell to produce electrical energy. Such metals include lead, zinc, iron, cadmium, aluminum and magnesium. Zinc is normally preferred because of its availability, energy density, safety, and relatively low cost.
A suitable electrolyte is an aqueous electrolyte including Group I metal hydroxides such as LiOH, NaOH, KOH, CsOH, or the like.
Metal-air battery cells are often arranged in multiple cell battery packs within a common housing to provide a sufficient amount of power output. The housing is necessary to seal-off the cells from the ambient air to prevent self-discharge of the cells during periods of non-use, which would result in a decreased battery output and lifetime. Because of the housing, however, it is necessary to provide a supply of oxygen to the cells when they are in use.
Typically, the oxygen is supplied by ambient air, which contains approximately 21% oxygen. The ambient air enters through ventilation holes in the housing that are open during cell use. In the housing, the ambient air is swept across the air cathodes of the cells as reactant air. As the reactant air crosses the air cathodes, the oxygen is depleted by reaction with the cells. After the reactant air has passed across the air cathodes of the cells, it is exhausted outside of the housing. Thus, during cell use, ambient air is drawn into the housing in a continuous flow that is sufficient to achieve the desired power output. Such an arrangement is shown in U.S. Pat. No. 4,913,983 to Cheiky, wherein a fan within the battery housing is used to supply a flow of ambient air to the air cathodes of the metal-air cells.
A problem associated with supplying oxygen from the ambient air, however, is that the humidity of the ambient air can cause a metal-air battery to fail. Equilibrium vapor pressure of the metal-air battery results in an equilibrium relative humidity that is typically about 45%. If ambient humidity is greater than the equilibrium relative humidity value for the metal-air battery, the metal-air battery will absorb water from the air through the cathode and fail due to a condition called flooding. Flooding may cause the battery to leak. If the ambient humidity is less than the equilibrium relative humidity value for the metal-air battery, the metal-air battery will release water vapor from the electrolyte through the air cathode and fail due to drying out. In most environments where a metal-air battery is used, failure occurs from drying out,
In the past, attempts have been made to solve the problems of flooding and drying out by controlling the flow of ambient air and reactant air. To gain more control over reactant air, one prior design separates the reactant air flow from a cooling air flow. In this design the reactant air flow rate is reduced relative to the cooling flow to reduce flooding or drying out effects. It has also been suggested prior to the present invention to control the humidity of air flowing into an air battery.
Specific examples of air managers that control the flow of reactant air are shown by U.S. Pat. No. 4,729,930 to Beal et al. and U.S. Pat. No. 4,913,983 to Cheiky, which is noted above. Beal discloses an apparatus for regulating and augmenting air supply for a fuel cell power plant during transient load increases. Beal provides a load monitor that has its output connected to the input of a microprocessor, the output of which adjusts a motor-controlled valve in the air supply line. In order to prevent oxygen starvation of the fuel cell when the imposed load increases Beal calculates the time needed for the control valve to reach a setting which allows an oxygen flow to meet increased load demand. According to Beal, when the oxygen supply cannot be increased through the control valve rapidly enough to immediately meet load demand, Beal provides auxiliary solenoid valves which open at the instant of increased load demand so as to provide oxygen to the fuel cell more quickly. The auxiliary valves will close when the difference in theoretical current produced by the available oxygen and the actual load demand drops below one or more preselected values.
While Beal discloses a method to increase oxygen supply when load demand increases, Beal does not disclose a method to decrease the oxygen supply to the fuel cell when load demand decreases so as to provide moisture control by limiting an excess amount of dry air or high humidity air to prevent drying out or flooding. Neither does Beal disclose recirculating the reactant air utilized by the cathode to provide moisture control.
U.S. Pat. No. 4,913,983 to Cheiky discloses a metal-air battery power supply to which the air flow is varied by a variable speed fan. The metal-air battery cells are enclosed in an air tight chamber which has a baffle that is movable in front of the air inlet and air outlet to seal off the container when the power supply is not in use. When the power is turned on, the air baffle is moved to permit an air flow into the air inlet that is exhausted through an air outlet. A fan which controls the air flow through the air inlets is run at different speeds depending upon use requirements of the connected computer. Cheiky, however, does not disclose limiting the air available through the air inlet to the air cathode for varying levels of output.
In addition to controlling the flow of reactant air, reactant air has been recirculated in the prior art in an attempt to maintain desired moisture for the reactant air entering a fuel cell. The recirculated reactant air, however, has become depleted of oxygen. In recirculating reactant air, a select amount of the recirculating air must be continually exchanged with ambient air. Because the humidity of the ambient air is not in equilibrium with that of the cells, the difference will tend to dry or flood the cells.
Therefore, there exists a need for a recirculating air manager that minimizes the amount of reactant air exchanged with ambient air to prevent flooding and drying out of metal-air batteries. Also, because the oxygen concentration of the air being recirculated is necessarily less than that of ambient air, the recirculation air manager should be able to ensure that the flow of recirculated air reaches the entire area of all the air cathodes. This ensures that all of the cells receive a sufficient amount of oxygen. Further, the recirculation air manager should be compact and lightweight such that the resulting battery remains relatively light and compact and thus can be easily used in conjunction with portable equipment.
Moreover, the recirculation air manager should be able to operate safely in conjunction with rechargeable metal-air batteries. Rechargeable metal-air batteries, as described above, generate hydrogen gas during recharging, which can be explosive at a high enough concentration in the presence of oxygen. In non-recirculating or one pass air managers, the hydrogen gas is not a problem because it is immediately exhausted with the reactant air. However if a recirculating air manager were to be used in conjunction with rechargeable metal-air batteries, the bulk of the hydrogen and oxygen gas generated from recharging would stay in the battery housing. Therefore, there exists a need for a recirculation air manager that can prevent the collection of the hydrogen gas generated by rechargeable metal-air batteries.
Specific examples of recirculation air managers are shown by U.S. Pat. No. 3,473,963 to Sanderson and French Patent No. 2,353,142 to Jacquelin. Sanderson discloses a system that provides cooling air and recirculated reactant air for a hydrogen and oxygen fuel battery. The cooling air is drawn into the battery casing by a large volume fan and blown through cell cooling chambers. On discharge from the cooling chambers, a portion of the used cooling air is mixed with recirculated reactant air and blown through reactant air chambers by a second fan. The proportion of used cooling air to recirculated reactant air is regulated by a plurality of control valves. The hydrogen fuel is piped into the battery from an outside supply.
Because the fuel hydrogen must be piped into the battery from an outside supply, the Sanderson battery is not portable. Further, the Sanderson system is impractical for portable batteries because it requires cooling chambers in addition to reactant air chambers, a plurality of fans, and a plurality of control valves to recirculate reactant air, all of which add considerable weight and bulk to a battery. Thus, the Sanderson system is not feasible for portable batteries which must be extremely light and compact. Moreover, Sanderson is not a rechargeable metal-air battery. Thus, Sanderson does not alleviate the problems discussed above.
French Patent No. 2,353,142 to Jacquelin discloses an air supply system for a zinc-air electrochemical generator that provides a recirculating path for the air used by the cell. The air manager is designed to avoid local accumulations of carbonate deposits and to vary the output of the cells by varying the amount of the incoming air (oxygen). Jacquelin discloses three modes of operation: maximum mode, slow motion mode, and a predetermined functioning mode. During the maximum mode of operation air is admitted from the atmosphere across the cathode and then exhausted through an outlet. During the maximum mode of operation, the air is not recirculated. A turbo exhauster is provided to ensure circulation of the air when air is being exhausted to the atmosphere. Operation during the maximum mode with no control to compensate for the relative humidity of the air is subject to the problems discussed above with flooding and drying out. During slow motion mode, no ambient air is admitted nor is any air exhausted, thus the oxygen supply is rapidly depleted from the air. This results in a low or slow mode of operation. During the predetermined functioning mode, fresh air is mixed with the air in proportions set by a mixing valve. The Jacquelin invention proposes to maintain a constant flow of gas on the electrodes to assure a good distribution of residual carbon dioxide on the entire surface of the electrodes. The zinc fuel is piped into the battery from an outside supply.
Because the zinc must be piped into the battery from an outside supply, the Jacquelin battery is also not portable. The Jacquelin battery is also not rechargeable. In addition, no method is provided to supply and vary air flow to meet varying load demands while reducing the effects of flooding or drying out. Furthermore, the air is supplied to an enclosed space containing a series of cells as well as pipes carrying the anode solution. Since all components of the cell are exposed to the air flow, one would expect cooling requirements to interfere with control of the air for the purpose of supplying oxygen, because often more air is required for cooling than is needed for the electro-chemical reaction. Thus, Jacquelin does not alleviate the problems discussed above.
Thus, there exists a need for a recirculating air manager that minimizes the amount of reactant air exchanged with ambient air to prevent flooding and drying out of metal-air batteries. Furthermore, there exists a need for a recirculation air manager for a portable rechargeable metal-air battery that circulates reactant air to the entire area of all the air cathodes and which prevents the accumulation of hydrogen gas on recharge.